THE PAD READOUT OF THE CERES RICH DETECTORS

The CERES/NA45 electron spectrometer uses two Ring Imaging CHerenkov d etectors with UV-sensitive gas counters of 2.84 m(2) and 0.42 m(2) siz e. The backplanes of these gas detectors consist of 53 800 (48 400) sq uare pads in a 2.74 mm (7.62 mm) grid which pick up induced signals of similar to 2 x 10(5) electrons from single-electron avalanches. Compa ct multi-channel preamplifier modules with low-noise VLSI chips are di rectly plugged onto the gas detector backplanes. The pad amplitudes ar e corrected for pedestal variations, digitized and transferred sequent ially at 14 MHz in chains of up to 4096 pads to the control room, Afte r zero suppression, the pad information is processed in a chain with p ipeline organization, performing a transformation to xy-coordinates an d branching part of the data to a trigger processor for fast ring find ing. With a simple mechanical concept and a multiplexed data transfer, this readout method offers a good signal-to-noise ratio at gas gains of a few 10(5), high spatial resolution on large areas and unambiguous reconstruction of multiple hits at a cost of less than or similar to 6 DM/channel.